Passivation layers are used in a wide range of semiconductor CMOS and bipolar devices and packages. The primary function of a passivation layer is to hermetically seal and/or electrically isolate semiconductor devices/circuits in a multi-layer stack.
There are two general types of passivating layers that are currently used. The first type comprises inorganic nitrides and oxides which are generally hard, chemically inert, and impervious to moisture. These materials exhibit an adequate thermal conductivity as well as a high electrical resistivity and a high dielectric breakdown strength that provides excellent electrical isolation. However, the first type of passivating layers usually require high temperature processing for their deposition (&gt;400 C.). Furthermore, in many cases the materials are non-transparent, thereby complicating certain processing steps involving lithography.
The second type of passivating layers comprise organic polymers, such as polyimide. In general these polymers are soft, partially transparent, are not totally impervious to moisture and certain solvents, acids, or bases, and are limited to use at low temperatures (&lt;350 C.). However, their ease of deposition makes them an attractive choice in a number of applications.
Both of the above types of passivation layers require standard photolithography processing if they are to be patterned. Patterning is necessary in many applications in order to make metal contacts through the passivation layer to connect the upper metal layer on microelectronics devices to layers below. However, patterning introduces a number of additional steps such as photoresist deposition, photolithography, and reactive ion or wet chemical etching to produce contact holes in the insulator and metallization to facilitate electrical connection. Further, the contact hole topography and edge profile can affect the extent to which the metallization conformally covers same, and can also affect the contact resistance and reliability of the interconnection between levels. While shallower wall profiles in thin insulator layers are generally most conducive to achieving good step coverage, steep wall profiles are required in order to achieve a high area density of contacts. Furthermore, providing a good dielectric breakdown voltage necessitates the use of thicker insulators. These conflicting requirements detrimentally limit the choice of metallization processes as well as an upper bound on practical contact densities in devices. Consequently, there is a long-felt need for a robust, transparent and impervious passivation layer that can be easily deposited, patterned and interconnected in fewer process steps. It is further desirable to achieve a contact hole and metal fill structure in a coplanar morphology to avoid the above-mentioned step coverage related issues.
Aluminum nitride is hard, robust, chemically inert, optically transparent, impervious to moisture, and exhibits a high thermal conductivity and electrical resistivity. That is, the use of AlN provides a passivation layer having excellent mechanical, thermal and dielectric properties. The physical properties of AlN are shown in Table 1.
TABLE 1 Properties of AlN Hardness 7 Mohs/1200 Knoop Resistivity 10.sup.13 Ohms/cm Dielectric Constant 8.5 Thermal Conductivity 0.3 W/cm K Solubility Impervious to most acids and bases
In addition, AlN has the property that when exposed to ultraviolet radiation above a certain power density, for example greater than 100 mJ/cm.sup.2, the nitrogen in the AlN preferentially desorbs leaving behind a thin film of Al. This property was observed in bulk AlN by Li et al., Mat. Res. Soc. Symp. Proc. 390, 257 (1995).
In U.S. Pat. No.: 5,225,251, issued Jul. 6, 1993, entitled "Method for Forming Layers by UV Radiation of Aluminum Nitride", H. Esrom describes a process for irradiating an aluminum nitride layer with ultraviolet radiation in the range from 240 nm to 270 nm, resulting in the elimination of the nitrogen component from the aluminum nitride for forming an aluminum layer. The aluminum layer is then reinforced with another metal using a metal deposition process.